A blue-violet semiconductor laser, made of Group III-V nitride semiconductors (AlxGayIn1-x-yN (where 0≦x≦1 and 0≦y≦1)) such as gallium nitride (GaN), is a key device for realizing ultrahigh density recording by optical disk drives, and is about to be actually used in consumer electronics products. The output of a blue-violet semiconductor laser must be increased not just to enable high-speed writing on an optical disk but also to develop new fields of technology including application to laser displays.
Recently, a GaN substrate has been regarded as a strong candidate for a substrate for fabricating a nitride semiconductor device thereon. This is because a GaN substrate is superior in the degree of crystal lattice matching and heat dissipation ability to a sapphire substrate that has been used in the pertinent art. Another advantage is that the GaN substrate has electrical conductivity, whereas the sapphire substrate is an insulator. That is to say, a structure in which current also flows across a GaN substrate can be adopted by arranging an additional electrode on the bottom surface of the GaN substrate, too. If an electrode is arranged on the bottom surface of a GaN substrate with electrical conductivity, then the size (i.e., the chip area) of each semiconductor device can be reduced, thus increasing the number of chips that can be made out of a single wafer. As a result, the manufacturing cost can be reduced.
Semiconductor lasers, including an n-electrode on the bottom surface of a GaN substrate, are disclosed in Patent Documents Nos. 1 to 3, for example.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2002-16312        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2004-71657        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2004-6718        